The present invention relates generally to security systems used in residential, commercial, industrial and government applications. More specifically, the present invention provides an FCC Part 15 compliant multi-frequency wireless transmitter for use in security systems.
Security systems have steadily increased in complexity over the years, beginning with the simple lock to the modern electronic security systems. Current security systems are not only designed to protect a home or commercial property from unauthorized intrusion but also to provide status of environmental conditions, such as temperature, air quality, fire warnings, carbon monoxide warnings, etc. Such systems include a myriad collection of sensors ranging from video cameras, infrared sensors, motion detectors, pressure sensors, temperature sensors, smoke detectors, and various air quality sensors. These sensors are distributed throughout a property and usually linked to a centralized security monitoring system.
These sensors are in communication with the monitoring system by various methods. Hard wiring is the most common means, wherein the sensors are physically wired to the monitoring system. However, hard wiring numerous sensors can be daunting when the security system is being installed in an already established site. Such wiring would necessitate cutting through walls to pass the wires from the individual sensors to the monitoring system.
This problem, for the most part, has been overcome with the advent of wireless communication technologies utilizing Radio Frequency (RF). RF frequencies generally extend from about 9 KHz to well over 100 GHz in the electromagnetic spectrum—frequencies above 3 GHz are generally referred to as microwave spectrum. The RF range of frequencies includes bands used for broadcast television (VHF, UHF), cordless phones (MHz-GHz range), cellular phones (GHz), wireless networking, remote control devices, etc. Usage of RF bandwidth is, largely, regulated by the FCC to ensure safe and interference-free operation.
The RF band of particular interest for communication between sensors and a monitoring system are in the 260 MHz to 470 MHz range. These frequency bands are regulated by Part 15 of the FCC Regulations, specifically 47 C.F.R. § 15.231. Under the Part 15 rules, these RF bands are available to the public without any licensing requirements, however there are strict guidelines that must be followed nonetheless. For example, power output must not exceed certain limits which are dependent on the specific broadcast frequency—e.g., for a frequency of 260 MHz, the radiated field strength shall not exceed 3,750 μV/m, while for a frequency of 470 MHz the radiated field strength shall not exceed 12,500 μV/m. The radiated field strength for the remaining frequencies lying between 260 MHz and 470 MHz can be calculated by interpolation. The complete rules governing the use of these frequency bands are available in 47 C.F.R. Part 15 published by the U.S. Government Printing Office.
RF transmitters incorporated into security system sensors and remote activation devices transmit in one of two ways. The simplest transmission method employs a single frequency as a carrier frequency. However, single frequency transmission is susceptible to interference from background noise produced by other RF devices.
An alternative transmission method relies on what is called frequency hopping, also known as spread spectrum. In frequency hopping, many frequencies are used to transmit a signal. While frequency hopping offers more reliability over single frequency, frequency hopping is more complex and costly to implement. The basic idea behind frequency hopping is simple—instead of transmitting on one frequency, a frequency hopping system switches rapidly from one frequency to the next. The choice of the next frequency is random, so it is nearly impossible for someone to eavesdrop or jam the signal. In frequency hopping, only packets containing a small portion of a message are transmitted on each frequency. However, keeping both the transmitter and receiver synchronized is the challenge, which requires accurate clocks and pseudo-random number generators.
A need exists for an RF transmitter with better reliability than single frequency RF transmitters, while still being inexpensive and relatively simple in design compared to frequency hopping RF transmitters, but yet be in compliance with FCC Part 15 regulations.